The present invention relates to an apparatus for measuring a small force, and more particularly relates to an electrocapacitive type force measuring apparatus in which an applied force is measured by detecting a change in an electrostatic capacitance.
In the above mentioned electrocapacitive type force measuring apparatus, a measuring sensitivity becomes highly inversely proportional to a spacing of the gap, and therefore in order to measure a very small force with a high sensitivity, it is necessary to set a spacing of the gap to a very small value such as 1-100 xcexcm. In the known electrocapacitive type force measuring apparatus, in order to attain such a fine gap, one of the electrodes is secured onto a flat surface of the base member via a spacer formed by a polymer film having a thickness which is identical with a gap distance to be realized. However, a thickness of such a polymer film is changed in accordance with temperature variation, humidity variation and secular variation, which is a variation or change in a property of a device during an elapse of rather long time, and therefore a spacing of the gap could be maintained accurately. This results in an instability of measuring sensitivity and precision, and a reliable measurement could not be performed.
Japanese Patent Application Laid-open Publications Nos. 4-249726 and 4-299227 disclose apparatuses for measuring a small force or displacement by detecting a change in capacitance due to an applied force. In these known apparatuses, a fine gap is formed between a base member and a resiliently deforming member which can be displaced by a force to be measured and is supported by the base member, a pair of electrodes are arranged on these members to be opposed to each other via the fine gap, and an electrostatic capacitance of a capacitor formed by these electrodes is detected to measure an amount of the applied force or a displacement caused by the applied force.
In the above mentioned electrocapacitive type force measuring apparatus, a measuring sensitivity becomes high inversely proportional to a spacing of the gap, and therefore in order to measure a very small force with a high sensitivity, it is necessary to set a spacing of the gap to a very small value such as 1-100 xcexcm. In the known electrocapacitive type force measuring apparatus, in order to attain such a fine gap, one of the electrodes is secured onto a flat surface of the base member via a spacer formed by a polymer film having a thickness which is identical with a gap distance to be realized. However, a thickness of such a polymer film is changed in accordance with temperature variation, humidity variation and secular variation, and therefore a spacing of the gap could be maintained accurately. This results in an instability of measuring sensitivity and precision, and a reliable measurement could not be performed.
In order to increase a rigidity of the gap forming structure, there has been further proposed to form the resiliently deforming member and base member as a single integral body. Such an integral body can be manufactured by etching a silicon or germanium wafer utilizing the well established processes in the manufacture of semiconductor devices. However, it is very difficult to form the resiliently deforming portion having a length of several tens millimeters such that a part of the resiliently deforming portion is accurately opposed to a part of the base portion with a fine gap of the order of micron meter. Therefore, the measuring precision could not be increased sufficiently, and further a dynamic range is narrow.
For instance, in case of measuring an amount of expansion and compression of a protein sample caused by binding with ligand by the force measuring apparatus, a front end of the resiliently deforming member is coupled to one end of a force detecting probe whose other end is connected to one end of a sample whose other end is fixed. In this case, the probe has a needle-like tip and this tip is pierced into the protein sample. During this operation, a relatively large force is applied to the resiliently deforming member via the probe. Since the rigidity of the resiliently deforming member of the known force measuring apparatus is not sufficiently high, the resiliently deforming member is damaged by such a large force, and in an extreme case, the resiliently deforming member might be broken.
Furthermore, in the known electrocapacitive type force measuring apparatus, in order to measure a change in capacitance of the capacitor constructed by the electrodes arranged to be mutually opposed via the gap, the capacitor is connected in one side of a capacitance bridge circuit or is connected to constitute a resonance circuit together with an inductor. In any case, in the known electrocapacitive type force measuring apparatus, since the capacitance variation of the capacitor is detected by means of an analog circuit, it is difficult to conduct a stable measurement as well as to attain a wide dynamic range due to various factors.
Moreover, the known electrocapacitive type force measuring apparatus is influenced by variations in conductivity, dielectric constant and permeability of air, and a precise measurement could not be carried out under various conditions. That is to say, a water adsorption layer is naturally existent on surfaces of the electrodes constituting the capacitor, and a thickness of this water adsorption layer is changed in accordance with variations in temperature and humidity of a surrounding atmosphere and a capacitance is liable to be unstable. Furthermore, coils and feedback transformer provided in a measuring circuit are also subjected to a variation in temperature, and a stable measurement could not be performed.
Therefore, the present invention has for its object to provide an electrocapacitive type force measuring apparatus for conducting a stable and highly precise measurement with aid of a sensor unit, in which a rigidity of a resiliently deforming portion and a base portion constituting a capacitor can be increased and a distance between electrodes can be accurately maintained in the order of micron meter.
It is another object of the present invention to provide an electrocapacitive type force measuring apparatus, in which a minute variation in the capacitance of the capacitor can be measured in a highly precise and stable manner over a wide dynamic range by means of a measuring circuit which can detect a very small capacitance change accurately and stably by a digital process.
It is still another object of the invention to provide an electrocapacitive type force measuring apparatus, in which an accurate and stable measurement can be performed by reducing variations in humidity and temperature of an atmosphere surrounding sensor unit and measuring circuit.
According to the invention, an electrocapacitive type force measuring apparatus comprises:
a sensor unit including an integral body made of a hard material, said integral body having a resiliently deforming portion which is deformed in a bending fashion by an application of a force to be measured and has a first surface, and a base portion having such a rigidity that the base portion is not deformed by said force and having a second surface which is opposed to said first surface of the resiliently deforming portion via a gap;
first and second electrodes provided on said first and second surfaces of the resiliently deforming portion and base portion of said sensor unit, respectively;
first and second input terminals connected to said first and second electrodes, respectively; and
a measuring circuit connected to said first and second input terminals and including an inductor which is connected to a capacitor formed by said first and second electrodes in a serial or parallel manner to form a resonance circuit, said force applied to said resiliently deforming portion being measured as a change in a resonance frequency of said resonance circuit.
In the electrocapacitive type force measuring apparatus according to the invention, since the resiliently deforming portion and base portion are formed as a single integral body made of a hard material, the influences of temperature variation, humidity variation and secular variation, to which the known electrocapacitive type form measuring apparatus having the space made of a soft material arranged between the resiliently deforming portion and the base portion is subjected, can be reduced, and therefore a very small force or displacement can be measured accurately with a very high precision.
In a preferable embodiment of the electrocapacitive type force measuring apparatus according to the invention, said resiliently deforming portion includes a first fitting surface formed at one end of the resiliently deforming portion, said base portion includes a second fitting surface which is coupled to said first fitting surface and is in parallel with said second surface of the base portion, and said second surface of the base portion is retarded from the second fitting surface by a distance which is equal to a distance of said gap. By constructing the sensor unit in this manner, it is possible to obtain easily the fine gap having a distance of, for instance 1-100 xcexcm by polishing the second surface and second fitting surface of the base portion such that these surfaces become coplanar, and then polishing only the second surface to be retarded over a distance identical with a given gap distance.
Furthermore, in the electrocapacitive type force measuring apparatus according to the invention, said resiliently deforming portion and base portion of the sensor unit may be preferably made of a material having low conductivity and small coefficient of thermal expansion such as glass and ceramics, particularly fused quartz.
In a preferable embodiment of the electrocapacitive type force measuring apparatus according to the invention, a first conductive pad connected to said first electrode is provided on the first fitting surface of said resiliently deforming portion, and a second conductive pad connected to said first input terminal is provided on said second fitting surface of the base portion, said resiliently deforming portion is coupled to said base portion by means of said first and second conductive pads to connect electrically said first electrode to said first input terminal. In this structure, the first input terminal for connecting said first electrode provided on the resiliently deforming portion to said measuring circuit can be provided on the base portion, and therefore a conductive lead wire for connecting the first input terminal to the measuring circuit is not connected to the resiliently deforming portion, but to the base portion. Then, a displacement of the resiliently deforming portion can be prevented from being influenced by the conductive lead wire, and a much more accurate measurement can be performed.
When the electrocapacitive type force measuring apparatus according to the invention is applied to the measurement of a deformation or displacement of the above mentioned protein sample, the probe holding the sample can be connected to a front end of the resiliently deforming portion of the sensor unit. In this case, since the resiliently deforming portion has a large rigidity, the resiliently deforming portion is not deformed too much upon piercing the probe into the protein sample, and thus the resiliently deforming portion can be effectively prevented from being damaged.
In a preferable embodiment of the electrocapacitive type force measuring apparatus according to the invention, said measuring circuit comprises an oscillation circuit including said resonance circuit composed of the capacitor and inductor, and a digital frequency change detecting circuit for detecting in a digital manner a change in a frequency of an oscillation signal generated from said oscillation circuit, said frequency being substantially equal to the resonance frequency. By means of such a digital frequency change detecting circuit, a very small frequency change can be detected accurately and stably over a wide dynamic range.
Said frequency change detecting circuit may be constructed by a digital frequency counter for counting the oscillation signal for a predetermined time period, and a digital signal processing circuit for measuring the force applied to the resiliently deforming portion by processing a count value obtained by conducting the counting over a given time period, or may be constructed by a digital frequency counter for counting the oscillation signal, a circuit for measuring a time period during which a count value of the digital frequency counter reaches a predetermined value, and a digital signal processing circuit for measuring the force applied to said resiliently deforming portion by processing an output signal from said time period measuring circuit. An active element of the oscillation circuit may be formed by a semiconductor element having a low input capacitance such as MES-FET, J-FET and MOS-FET.
Moreover, in a preferable embodiment of the electrocapacitive type force measuring apparatus according to the invention, at least parts of said sensor unit and measuring circuit are installed within a housing such that they are isolated from an ambient atmosphere, and a dry air is circulated through the housing. By continuously circulating the dry air through the housing, undesired variations of temperature and humidity of the atmosphere within the housing can be suppressed and a thickness of water adsorption layers formed on the surfaces of the electrodes constituting the capacitor can be kept substantially constant. Therefore, the measurement can be performed stably.